Purified preparations of chitin synthetase (EC 188.8.131.52; UDP-2-acetamido-2-deoxy-D-glucose:chitin 4-beta-acetamidodeoxyglucosyltransferase), capable of forming microfibrils in vitro, were isolated from yeast cells of Mucor rouxii. Chitin synthetase was obtained either by substrate-induced liberation of bound enzyme (54,000 x g pellet) or by isolation of unbound enzyme present in the 54,000 x g supernatant of a cell-free extract. Both preparations contained ellipsoidal granules from about 350 to 1000 A diameter. Many granules exhibited a marked depression. No typical unit membrane profiles appeared in thin sections of glutaraldehyde/OsO4-fixed samples. Upon incubation with substrate and activators, chitin microfibrils were produced. The microfibrils were often found intimately associated with granules. The most common configurations were: a microfibril with a granule at one end, or two microfibrils "arising" from the same granule. These findings lend support to the granule hypothesis for the elaboration of cell wall microfibrils by end-synthesis.